Position encoders are commonly used to determine the linear or angular displacement of an object. One of many applications requiring high precision is telescope axis encoding. In telescopes of large size such as those utilized by observatories, telescope axis encoding is a significant problem due to the fine resolution desired and large dynamic range of measurement. Generally, such telescopes are mounted on a large disk or base rotatable about a vertical axis. The mounting of the telescope on the disk is such that the telescope is rotatable about a horizontal axis for altitude positioning. The axis encoding problem manifests itself, for example, when an astronomer is following a star in the sky and attempts to move the large telescope to track the movement of the star using a control system and a mechanical drive mechanism. If the telescope is moved slightly out of range, the star may move out of the field of view of the telescope and may not be seen. The problem is how to determine the actual movement of the telescope as produced by the mechanical drivers and whether that movement is in accordance with the desired movement necessary for proper tracking of the star as calculated by the control system.
There are several existing encoders that incorporate different methods for measuring the actual displacement of an object. However, all suffer from various problems that make them undesirable, particularly for applications that require high precision. A common device for position measurement is a friction driven encoder in which a rotary encoder is coupled to a capstan. The capstan is then driven by a large diameter surface to achieve the desired position, or angular resolution. These devices can be serviceable; however, they involve difficult mechanical tolerances due to roller slippage and other problems and are dirt sensitive.
Another alternative utilized to measure the displacement of the telescope is a tape encoder. One tape encoder, produced by Heidenhain, uses a read head for reading patterns on a tape coupled to the base of the telescope. The tape patterns are light and dark lines spaced apart at approximately 100 microns. The patterns move along the head, whereby the head determines displacement based on the read patterns. The problem with this tape encoder is that it is dirt sensitive and has difficult alignment tolerances. Another tape encoder, produced by Farrand, is an inductive tape encoder in which the head abuts the tape. The Farrand tape has a copper trace pattern through which an ac signal is passed. The head picks up a magnetic field caused by the current in the copper trace and measures offset based on magnetic field strength. The inherent problem with this system is that the electrical and magnetic fields are affected by electrical interference from other wires or machinery in the room. This may introduce unacceptable error. These tape encoders also involve difficult mechanical and electrical details, can be dirt sensitive, and can be rather expensive.
Other applications use glass scale encoders in which a reflective pattern is put on glass. One example of such an encoder, produced by Sony and Cannon, is called a holoscale. Finely spaced pattern lines cause diffraction that can be measured by a tracking system. Problems with these devices include sensitivity to dirt, difficult alignment tolerances, and relatively high cost compared to other available devices.
Another position encoding technique known in the art is used at the U.S. Naval Observatory at Flagstaff, Ariz. There, a video camera images a scale attached around the circumference of the telescope base. Mathematical operations are used to locate the image in relation to a predetermined calibration point. This technique is not well suited for tracking moving objects because of the required processing time of the mathematical correlation.
Finally, another encoding system, described in Spies, U.S. Pat. No. 4,595,991, determines the position of an object through the use of scanning elements and harmonic-free periodic signals. The periodic analog signals generated by the scanning elements are subject to a Fourier analysis to determine the Fourier coefficients of the fundamental wave of the signals. The use of a Fourier analysis as part of a mathematical algorithm to determine the position of an object reduces the required processing time of the mathematical correlation. These Fourier coefficients are then evaluated as harmonic-free periodic signals for the formation of position measuring values. While this technique teaches the extraction of a frequency and finding its phase, it relies on only a relatively small number of samples. The samples are obtained using six separate scanning elements arranged in a cumbersome manner. Extending this device to use even 100 samples would be very unattractive from a practical standpoint. Furthermore, the invention is expensive to manufacture because of the close tolerances and alignments caused by its photodetector scanning element arrangement. The close mechanical tolerances and precise apertures involved in the arrangement also render the device sensitive to contamination.
Therefore, there is a need for an improved position encoder which is noncontacting, relatively less sensitive to dirt contamination and line to line spacing variations, and which has practical alignment tolerances. A desirable encoder should not involve difficult mechanical and electrical details, should not have electrical and magnetic fields which are affected by electrical interference from other wires or machinery in the room, and should be relatively low in cost compared to those in the prior art. Finally, the required processing time of any mathematical operation used should be relatively short so as to allow the device to be useful for real-time tracking of the displacement of a moving object, e.g., the moving base of a telescope.